Many applications, particularly in the field of wavelength-division-multiplexing (WDM) communication systems, require wavelength-tunable optical pulse sources with multigigahertz repetition rate.
Fiber optical parametric oscillators in the form of a modulational instability laser are known in which all of the pump, signal and idler waves are resonated inside a fiber ring cavity. The operation of this system, however, is undesirably phase sensitive because all three of the waves are resonant. Known fiber optical parametric oscillators have oscillated at the point of modulational instability where the gain of the signal is at a maximum. However, the oscillation frequency can be shifted by only a small amount by changing the pump power. They are therefore, not tunable to a desired frequency over the gain bandwidth.
Another known fiber parametric oscillator is disclosed in U.S. patent application Ser. No. 09/431,550 entitled “Tunable Fiber Optic Parametric Oscillator” filed Oct. 29, 1999. This fiber optic parametric oscillator includes a fiber parametric amplifier in the form of a nonlinear fiber Sagnac interferometer or a nonlinear optical loop mirror having a pump wave port and a signal wave port with a dispersion shifted fiber portion therebetween for amplifying a signal wave having a signal pulse coincident with a pump wave pulse and simultaneously generating an idler wave. The pump wave is reflected back to the pump wave port whereas the signal and idler waves pass through the signal port and are directed to a spectral filter in the form of a diffraction grating. The spectral filter is tunable to reflect a signal wave of a particular wavelength back to the fiber parametric amplifier so as to provide nondegenerate operation of a synchronously pumped fiber optic parametric oscillator. However, this parametric oscillator needs a powerful source and extremely long lengths of fiber.
Microstructure fibers having a high nonlinearity per unit length and low susceptibility to bending loss are known. Optical generation schemes based on nonlinear parametric effects in fibers are beneficial for this purpose for two reasons: 1) the wavelength tunability of the oscillating signal is not limited by the erbium-doped fiber amplifier (EDFA) gain band and 2) the fundamental limit on the repetition rate is the time response of the fiber's Kerr nonlinearity, which is in the femtosecond range. A simple means of implementing such a source relies on pulsed supercontinuum generation (SCG) in the fiber followed by spectral slicing. This scheme, however, suffers from reduction of the power spectral density as the wavelength detuning between the desired signal and the pump is increased. Furthermore, it has been shown that the coherence of the generated supercontinuum is limited; it leads to progressive degradation of the spectral modulation depth of the signal pulses as one moves away from the pump wavelength.